Learn what is clousers in Javascript

The closure has three scope chains:

function outer() {
  let i = 1;
  return function inner() {
    const a = 10;
    const result = a * i;
    i = i + 1;
    return result;
  };
}

const inner = outer(); //get instance of inner function

console.log(inner()); // 10
console.log(inner()); // 20
console.log(inner()); // 30

 

Use:
Mostly, closures are used for encapsulation in javascript. You can declare private variables using closures. These variables will be hidden for other functions/methods. Consider another example of closure as:

function counter() {
  var c = 0;
  return {
    inc: function () {
      ++c;
    },
    dec: function () {
      --c;
    },
    get: function () {
      return c;
    },
    reset: function () {
      c = 0;
    },
  };
}

const myCounter = counter();

console.log(myCounter.inc());
console.log(myCounter.get()); //1

console.log(myCounter.dec());
console.log(myCounter.reset());

Inner variables and functions of counter function are hidden from other functions. Using closure you can encapsulate your private methods. Consider another example:

function calculate() {
  const rate = 5;
  return {
    intrest: function (principal, time) {
      return (principal * rate * time) / 100;
    },
    totalAccruedAmount: function (principal, time) {
      return principal + (principal * rate * time) / 100;
    },
  };
}

const calculator = calculate();

console.log(calculator.intrest(1000, 5));
console.log(calculator.totalAccruedAmount(1000, 5));

In this example interest rate is private and only closure functions can access interest rate. It is hidden from the outer scope and cant be accessed by other scopes. 

You can use closures to hide implementation details and declare private variables.